Swelling clays can be a major mechanism of formation damage due to loss of mobility of hydrocarbon fluids in the formation. When clays encounter foreign water, they can swell, causing a loss of permeability. The swelling can cause portions of the clay and adjacent fines to become mobile within the production stream and, too frequently, encounter constrictions in capillaries, where they can bridge off the capillaries and severely diminish the flow rate of hydrocarbons to the wellbore. Sometimes the loss of permeability observed is due to clay swelling without migration, but often clay swelling is accompanied by migration of clay and other fines. Non-swelling clays may also respond to the foreign water and begin to migrate.
Shale is a fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and silt-sized particles into thin, relatively impermeable layers. Some shales encountered during subterranean operations can be sensitive to water, due in part to clay content and the ionic composition of the clay. Such shales, also known as heaving or sloughing shales, can have a tendency to degrade, such as swell or crack, upon contact with various downhole fluids, such as drilling fluids and fracturing fluids. The complications associated with shale degradation during drilling may substantially increase the time and cost of drilling. The degradation of shales in a borehole can render the borehole walls unstable. The heaving shale material can slough and cave into the borehole. Degradation of the shale can interrupt circulation of the drilling fluid and cause greater friction between the drill string and the wellbore. Sloughing of shale material into the borehole can cause the drill stem to become stuck and can enlarge the borehole, with the result that large subterranean cavities are formed. The degradation of the shale may interfere with attempts to maintain the integrity of drilled cuttings traveling up the well bore until such time as the cuttings can be removed by solids control equipment located at the surface. Degradation of drilled cuttings prior to their removal at the surface may prolong drilling time because shale particles traveling up the well bore can break up into smaller and smaller particles, which can expose new surface area of the shale particles to the drilling fluid and lead to further absorption of water and degradation. Where sloughing occurs while the drilling bit is being changed at the surface, the borehole fills up and must be cleared before drilling can proceed. The heaving shale material taken up into the drilling fluid can adversely affect the viscosity characteristics of the drilling fluid to the point where the fluid must be chemically treated to reduce the viscosity thereof or it must be diluted followed by the addition of weighing material to maintain a given mud weight.
Using oil-based fluids instead of aqueous-based fluids is one method of inhibiting clay swelling and shale degradation. However, oil-based fluids are often environmentally undesirable because they may be toxic. Accordingly, environmental regulations enacted by numerous countries have curtailed the use of oil-based fluids. Another method is to use clay or shale stabilizers. However, many clay or shale stabilizers are expensive and are environmentally undesirable due to toxicity or lack of biodegradability, and can cause damage to subterranean formations.